Allotropic form

In the preceding sections, the various ways that carbon atoms bond together to form solids were reviewed. These solids are the allotropes (or polymorphs) of carbon, that is, they have the same building block—the element carbon—but with different atomic hybrid configurations sp (tetragonal), sp (trigonal) or sp (digonal). 

These allotropic solids can be classified into three major categories (i) the sp structures which include graphite, the graphitic materials, amorphous carbon, and other carbon materials (all reviewed in Ch. 3), (ii) the sp structures which include diamond and lonsdaleite (a form detected in meteorites), reviewed in Ch. 11, and (iii) he Fullerenes (see Ch. 15). 

These allotropes are sometimes found in combination such as some diamond-like carbon (DLC) materials produced by low-pressure synthesis, which are actually mixtures of microcrystalline diamond and graphite (see Ch. 14). 

Recent investigations have revealed the existence of a series of diamond polytypes such as the 6-H hexagonal diamond. The structure and properties of these polytypes are reviewed in Ch. 11Also under investigation is a hypothetical phase of carbon based on a three-dimensional network but with sp bonds. This phase could be harder than diamond, at least in theory.1 A carbon phase diagram incorporating these new polytypes has yet to be devised. 

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None of the common allotropic forms of these metals is affected by air unless they are heated, when aU burn to the (III) oxide. 

Oxygen, sulphur and selenium are known to exist in more than one allotropic form. 

This exists in two allotropic forms, oxygen, O2 and ozone, O3. 

Like sulphur, selenium exists in a number of allotropic forms. These include both crystalline, rhombic and monoclinic modifications... 

Carbon is found free in nature in three allotropic forms amorphous, graphite, and diamond. A fourth form, known as "white" carbon, is now thought to exist. Ceraphite is one of the softest known materials while diamond is one of the hardest. 

In 1824 Berzelius, generally credited with the discovery, prepared amorphous silicon by the same general method and purified the product by removing the fluosilicates by repeated washings. Deville in 1854 first prepared crystalline silicon, the second allotropic form of the element. 

Phosphorus exists in four or more allotropic forms white (or yellow), red, and black (or violet). Ordinary phosphorus is a waxy white solid when pure it is colorless and transparent. White phosphorus has two modifications alpha and beta with a transition temperature at -3.8oC. 

Sulfur is pale yellow, odorless, brittle solid, which is insoluble in water but soluble in carbon disulfide. In every state, whether gas, liquid or solid, elemental sulfur occurs in more than one allotropic form or modification these present a confusing multitude of forms whose relations are not yet fully understood. 

Selenium exists in several allotropic forms. Three are generally recognized, but as many as that have been claimed. Selenium can be prepared with either an amorphous or crystalline structure. The color of amorphous selenium is either red, in powder form, or black, in vitreous form. Crystalline monoclinic selenium is a deep red crystalline hexagonal selenium, the most stable variety, is a metallic gray. 

Strontium is found chiefly as celestite and strontianite. The metal can be prepared by electrolysis of the fused chloride mixed with potassium chloride, or is made by reducing strontium oxide with aluminum in a vacuum at a temperature at which strontium distills off. Three allotropic forms of the metal exist, with transition points at 235 and 540oC. 

The metal has a bright silvery metallic luster. Neodymium is one of the more reactive rare-earth metals and quickly tarnishes in air, forming an oxide that spalls off and exposes metal to oxidation. The metal, therefore, should be kept under light mineral oil or sealed in a plastic material. Neodymium exists in two allotropic forms, with a transformation from a double hexagonal to a body-centered cubic structure taking place at 863oC. 

Ytterbium has a bright silvery luster, is soft, malleable, and quite ductile. While the element is fairly stable, it should be kept in closed containers to protect it from air and moisture. Ytterbium is readily attacked and dissolved by dilute and concentrated mineral acids and reacts slowly with water. Ytterbium has three allotropic forms with transformation points at -13oC and 795oC. The beta form is a room-temperature, face-centered, cubic modification, while the... 

Table 10. Properties of Allotropic Forms of Phosphorus(V) Oxide...

Ozone. Ozone is an allotropic form of oxygen, O. Because it is an unstable gas, it must be generated at the point of use. Ozone is an effective, clean oxidizing agent possessing powerful antibacterial and antiviral properties. 

A has three solid allotropic forms with change temperatures of 800°C and 1150°C and melts at 1980°C. These form solid solutions a, P and /containing B, a being the low-temperature one. 

W. Heisenberg (Leipzig) the creation of quantum mechanics, the application of which has, inter alia, led to the discovery of the allotropic forms of hydrogen. 

If you re a sports fan, you ve almost certainly seen this structure before. It is that of a soccer ball with a carbon atom at each vertex. Smalley and his colleagues could have named this allotropic form of carbon "carbosoccer" or "soc-cerene," but they didn t. Instead they called it "buckminster fullerene" after the architect R. Buckminster Fuller, whose geodesic domes vaguely resembled truncated soccer balls. 

The element phosphorus forms a variety of allotropic forms in the solid state. In the chemistry stockroom, you are likely to find red phosphorus and possibly white phosphorus (Figure B). As you can see, white phosphorus has the molecular formula P4, whereas red phosphorus might be represented as Px, where x is a very large number. The difference in properties between the two allotropes reflects the difference in their bonding patterns, molecular versus network covalent ... 

Dependence on Density.—If the density of a metal is increased by hammering, its specific heat is slightly decreased. The same change is observed if the change of density is due to a change of crystalline form, or to change from an amorphous state to a crystalline state, and with different allotropic forms (Wigand, loc. cit,). 

If two or more crystalline forms of a substance exist, they are called polymorphic forms if the substance is an element (C, S, P) they are called allotropic forms. The Thomson equation obviously applies to this case ... 

Abnormally low atomic heats were explained by Richarz on the assumption of a diminution of freedom of oscillation consequent on a closer approximation of the atoms, which may even give rise to the formation of complexes. This is in agreement with the small atomic volume of such elements, and with the increase of atomic heat with rise of temperature to a limiting value 6 4, and the effect of propinquity is seen in the fact that the molecular heat of a solid compound is usually slightly less than the sum of the atomic heats of the elements, and the increase of specific heat with the specific volume when an element exists in different allotropic forms. 

Allotropic Form Appearance or Crystal Form Mp, ° Bp,° Density, g/c... 

Titanium Dioxide (Titania, Titanium White, Rutile, Anatase, Brookite, Titanic Anhydride, Titanic Acid Anhydride, Titanic Oxide). Ti02, mw 79.90, Selected Parameters of the Allotropic Forms of Ti02... 

Investigations of the chemical properties of plutonium have continued in many laboratories throughout the world as it has become available. This has led to the situation where the chemistry of this relative newcomer is as well understood as is that of most of the well-studied elements. The four oxidation states of plutonium—III, IV, V, and VI—lead to a chemistry which is as complex as that of any other element. It is unique among the elements in that these four oxidation states can all exist simultaneously in aqueous solution at appreciable concentration. As a metal, also, its properties are unique. Metallic plutonium has six allotropic forms, in the temperature range from room temperature to its melting point (640 C), and some of these have properties not found in any other known metal. 

Ironis very reactive chemically and oxidizes readily. It has four allotropic forms, one of which (a) is magnetic with a Curie transition point of770°C. Ithasadensity of7.6g/cm, ameltingpointof 1536°C, athermal expansion of 12.6 ppm/°C at 25°C, a thermal conductivity of 0.80 W/ cm °C at25°C and an electrical resistivity of 9.71 iohm-cm at 20°C. 

There are more than a million known carbon compounds, of which thousands are vital to life processes. The carbon atom s unique and characteristic ability to form long stable chains makes carbon-based life possible. Elemental carbon is found free in nature in three allotropic forms amorphous carbon, graphite, and diamond. Graphite is a very soft material, whereas diamond is well known for its hardness. Curiosities in nature, the amounts of elemental carbon on Earth are insignificant in a treatment of the... 

Calcium is miscible with Sr in the liquid and in all the solid bcc, hep and fee allotropic forms (Fig. 1). Barium exhibits no hep or fee forms, however, so that solid solubility between the close-packed structures of Ca and Sr, and the bcc structure of Ba is restricted in the Ca-Ba and also in the Sr-Ba systems. A continuous series of solid solutions is only achieved in Ca-Ba and Sr-Ba for the high-T bcc modifications. In Ca-Ba, the solid solutions are separated by a narrow heterogeneous field between 32 and 36 mol% Ba in Sr-Ba this occurs between 24 and 30 mol% Ba (Fig. 1). 

The state of research on the two classes of acetylenic compounds described in this article, the cyclo[ ]carbons and tetraethynylethene derivatives, differs drastically. The synthesis of bulk quantities of a cyclocarbon remains a fascinating challenge in view of the expected instability of these compounds. These compounds would represent a fourth allotropic form of carbon, in addition to diamond, graphite, and the fullerenes. The full spectral characterization of macroscopic quantities of cyclo-C should provide a unique experimental calibration for the power of theoretical predictions dealing with the electronic and structural properties of conjugated n-chromophores of substantial size and number of heavy atoms. We believe that access to bulk cyclocarbon quantities will eventually be accomplished by controlled thermal or photochemical cycloreversion reactions of structurally defined, stable precursor molecules similar to those described in this review. 

The Nobel prize in Chemistry for the year 1996 was awarded for the discovery of the fullerenes, the third allotropic form of carbon, with Cgo and C70 as the two most prominent representatives. While the fullerenes of course are the epitome of carbon-rich molecular compounds, it is an irony that their synthesis is more of a physical phase transition, taking place under drastic conditions [1]. 

Basic physical properties of sulfur, selenium, and tellurium are indicated in Table 1.3. Downward the sulfur sub-group, the metallic character increases from sulfur to polonium, so that whereas there exist various non-metallic allotropic states of elementary sulfur, only one allotropic form of selenium is (semi)metallic, and the (semi)metallic form of tellurium is the most common for this element. Polonium is a typical metal. Physically, this trend is reflected in the electrical properties of the elements oxygen and sulfur are insulators, selenium and tellurium behave as semiconductors, and polonium is a typical metallic conductor. The temperature coefficient of resistivity for S, Se, and Te is negative, which is usually considered... 

Potassium violently reacts with carbon in most of its allotropic forms. Thus the action of molten metal on graphite results in the formation of some kind of a carbide ... 

When a test confirms the presence of a substance without determining the amount of substance present, the process is called qualitative analysis. In this activity, you will detect the presence of potassium in coffee by the characteristic yellow color that appears when potassium ions react with sodium hexanitrocobaltate. To make it easier to detect the yellow color, the coffee solution will be decolorized with charcoal, an allotropic form of solid carbon. 

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